Decoupling capacitor structure

ABSTRACT

A capacitor structure is described as having a plurality of dielectric materials located so that each dielectric material is in parallel between capacitor plates. The capacitor value of this structure is preset, therefore, for operation electrically at different specific temperatures. The description gives a specific stacked arrangement for the various dielectric materials in which this capacitor can be formed, as one example of that to which it is adaptable.

This is a continuation of Ser. No. 07/978,794 Nov. 19, 1992, nowabandoned.

BACKGROUND OF THE INVENTION

The present invention, generally, relates to electrical capacitorstructures and, more particularly, to a new and improved structuralarrangement for a capacitor which admits of functioning in a widertemperature range than prior structures.

In the past, no single capacitor could be used in an environment thatinvolved wide temperature variations. It is known that changes inoperating temperature can produce changes in the value of a capacitor.

Also, there is a continuing effort in the data processing field todevelop equipment having less cost, greater operating speed and smallersize. The need for smaller sizes of capacitors produces a naggingproblem in the microelectronics area today, because there are so manycapacitors needed, and the problem arises due to the need to use twocapacitors in order to cover the wider ranges of environmentaltemperatures to which computer systems are exposed today.

In today's systems, a single capacitor that is required for operation inan environment that involves a temperature range, from the highertemperature of today's data processing circuits down to the much coldercryogenic circuits, can fall short in its operation. Usually, a singlecapacitor in this environment will be replaced with two capacitors,producing an undesirable duplication in that more space is required fortwo capacitors.

Modern ferroelectric chip capacitors have made it possible to producecapacitor values ranging from a few picofarads to a few microfaradsrequiring much less spacial size than is possible usingnon-ferroelectric material. However, it has been found that thedielectric constant of such ferroelectric capacitors changes appreciablyin response to changes in ambient temperature.

Microelectronic components used in high frequency operating circuitenvironments, particularly used in the switching of integrated circuits,can produce transient energy being coupled into other, unwanted circuitareas. This can be avoided by using a decoupling capacitor across thecurrent source. Even the beneficial effects of a decoupling capacitorconnected in this circuit can be affected adversely when temperaturevariations produce changes in the value of the capacitor.

An example of an early attempt at developing a decoupling capacitor witha better stability in a high frequency noise environment is themultilayer construction described in U.S. Pat. No. 4,667,267 toHernandez et al. While the structure described in that patent may beeffective to accomplish the purpose intended, it states that thedecoupling capacitor of that structural arrangement is affectedadversely and the value of the capacitor becomes unstable as temperaturechanges.

In U.S. Pat. No. 4,706,162, Hernandez et al. describes differentconstructions for a multilayer capacitor in order to overcomedifficulties with inductance and to fit within a small space in anintegrated circuit. Here, again, it is admitted that capacitance valueschange as temperature changes, and the decoupling capacitor is affectedadversely, becoming unstable, as temperature changes.

U.S. Pat. No. 4,831,494 to Arnold et al., which is assigned to the sameAssignee as the present invention, describes a multilayer capacitorstructure that reduces the internal inductance even further, is smallerin size and is adapted to easier manufacturing techniques. However, itis not concerned with capacitor functioning effectively over a widerange of operating temperatures.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide astructural arrangement for a decoupling capacitor which providessubstantially stable functioning over a wider range of operatingtemperature.

It is also an object of the present invention to provide multipledielectric materials within the same capacitive unit for maintainingdecoupling effectiveness over a wide range of operating temperatures.

Briefly, a decoupling capacitor structure in accordance with the presentinvention includes two capacitor plates and at least two dielectricmaterials having predetermined dielectric constants that are differentfrom each other and arranged so that each dielectric material isconnected electrically with each of the capacitor plates.

The above, other and further objects, advantages and features of thepresent invention will become more readily apparent from the followingdetailed description of the presently preferred embodiment asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical capacitor constructed andarranged according to the present invention.

FIG. 2 is a curve illustrating capacitive characteristics vs.temperature for one dielectric in FIG. 1.

FIG. 3 is a curve illustrating capacitive characteristics vs.temperature for another of the dielectrics in FIG. 1.

FIG. 4 is an illustration of a capacitor structure that is arranged inaccordance with a modification of the invention.

FIG. 5 is an illustration of a further modified structure of thecapacitor of the present invention.

FIG. 6 illustrates another method of manufacturing the capacitorarrangement of FIG. 4.

FIG. 7 is a view in perspective illustrating diagrammatically anarrangement of a capacitor of the invention.

FIG. 8 is a curve showing the variations of capacitance values vs.temperature for four different combinations of dielectric materials asan aid in describing the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a typical capacitor constructed in accordance with theprinciples of the present invention and is identified generally by thereference numeral 10. One capacitor plate 11 has an electricalconnection 12, and a second capacitor plate 13 has a connection 14, bywhich the capacitor 10 is connected both electrically and operatively inan electrical circuit.

Present day ferroelectric materials have made it possible to constructelectrical capacitors with values that range from a few picofarads tovalues measured in microfarads that take up much less spatial volumethan is possible using nonferroelectric, solid, dielectric materials.However, even though these ferroelectric materials, as available today,solve the problem relating to smaller spaces, they have a tendency tochange their dielectric constants appreciably in response to changes intemperature.

However, in accordance with the present invention, a capacitor 10 isconstructed to enclose multiple dielectric materials 15 and 16 withinthe same capacitive unit, and it has been discovered that an arrangementof these dielectric materials in accordance with the invention permitsthe capacitor 10 to maintain a decoupling effectiveness over a widerange of temperature changes, even from 77° K. to 300° K., without aneed to change components.

In other words, a capacitor 10 constructed in accordance with theinvention provides the capability of providing a more nearly constantvalue of capacitance over wide variations in temperature than with priorcapacitors.

In FIG. 1, a first dielectric material 15 can be, for example, bariumtitanate, or BaTiO₃, which permits effective decoupling at 300° K., orat room temperature, and a second dielectric material 16 can be, forexample, strontium titanate, or SrTiO₃, which permits effectivedecoupling at 77° K., which is the temperature of liquid nitrogen. FIG.2 and FIG. 3 illustrate the change in capacitance characteristics withchange in temperature for these two dielectric materials.

The first and second dielectric materials 15 and 16 are positionedadjacent each other and joined by a conventional joining process, suchas, for example, adhesive bonding or a co-firing process. This joiningis illustrated in FIG. 1 by a barrier 17. The same capacitor plates 11and 13 are common to both dielectric materials 15 and 16, in accordancewith the invention, which connects the dielectric materials in parallelelectrically.

The generally accepted relationship to determine the value ofcapacitance is as follows: ##EQU1## where:

C=capacitance;

K=dielectric constant;

A=electrode area; and

T=dielectric thickness.

To determine the total capacitance between capacitor plates for acapacitor such as FIG. 1 in accordance with the invention, arelationship as follows is used: ##EQU2## where:

C=the total capacitance;

K₁ =dielectric constant for dielectric 1;

K₂ =dielectric constant for dielectric 2;

T=dielectric thickness;

A₁ =electrode area with dielectric 1; and

A₂ =electrode area with dielectric 2.

A dielectric barrier, such as the barrier 17, FIG. 1, is used to preventa diffusion of dielectric materials during firing, when a manufacturingprocess is used that involves a firing of the materials to fabricate thecapacitor. Such a firing can cause a diffusion between the twodielectric materials, thereby producing a third dielectric material ofunpredictable value, and therefore, the total capacitance value of thecapacitor becomes unpredictable.

FIG. 4 of the drawings illustrates another form of construction for acapacitor that utilizes the features of the present invention. Thisarrangement has capacitor plates 40 and 41 and dielectric 1 divided intotwo parts 42 and 43 positioned on opposites of dielectric 2 that isidentified also by the reference numeral 44.

To understand and appreciate the reason for dividing one of thedielectric materials this way, note that in relationship (2) above, theareas A₁ and A₂ are two of the variables. Therefore, a largerproporation of the electrode area will accommodate more volume of eitherof the dielectric materials.

FIG. 5 illustrates a different arrangement of the capacitor of theinvention where plates 50 and 51 have dielectric 1 divided into parts 52and 53 and dielectric 2 divided into parts 54 and 55 with a thirddielectric 3 positioned in the middle and identified by the numeral 56.Even with multiple dielectric materials, it should be observed that eachdielectric material is in contact with each of the capacitor plates,thereby forming electrically one capacitive value for each dielectricmaterial, as illustrated by FIGS. 2 & 3 of the drawings.

The division of dielectric materials into multiple parts has no adverseeffect for the capacitance value between the plates. But with anarrangement according to the invention, i.e., each dielectric materialof whatever kind being in contact electrically with both capacitorplates, there is no limitation to making the capacitor by any particularprocess.

Therefore, it will be understood that the capacitor of the invention mayhave any number of dielectric materials between the capacitor plates,depending upon the temperatures at which the capacitor is to functioneffectively, and the arrangement of these dielectric materials is suchthat they each are in contact electrically with the capacitor plates.Any division of the dielectric materials is dependent upon theparticular manufacturing process that is used.

FIG. 6 illustrates the capacitor arrangement of the present inventionwith multiple dielectric materials when a firing process is not used. Inthis instance, the capacitor plates are identified by the numerals 60and 61, and a first dielectric material is identified by the numerals 62and 63. A second dielectric material 64 is positioned to be inelectrical contact with both capacitor plates 60 and 61, which is aparallel arrangement with the dielectric material 62 and 63.

A low modulus adhesive 65 and 66 separates the respective dielectricmaterials, as illustrated by FIG. 6.

With a capacitor structure in accordance with the invention, anarrangement as shown in perspective in FIG. 7 illustrates theflexibility that is available. In this stacked capacitor, or it may becalled a "piggy back" capacitor, arrangement, a substrate 70 is shownwith pads 71 arranged according to a needed connection for connecting acircuit, not visible, with a decoupling capacitor to functioneffectively at a particular temperature.

The capacitor of the invention, as illustrated in FIG. 1, is constructedin the arrangement of FIG. 7, with a first dielectric material 72 and asecond dielectric material 73 with capacitor plates 74 arranged inelectrical contact with respective pads 75 and bumps 76. With thecapacitor of the invention constructed in this manner, any number oflevels of different dielectric materials can be formed according to thecapacitance value needed for a circuit function.

FIG. 8 shows several curves of particular capacitance values offerroelectric chip capacitors that are constructed in accordance withthe invention, i.e., a single capacitor capable of functioning effectiveat two different temperatures. Curve A shows capacitance vs. temperatureusing a single dielectric alone, such as Barium Titanate, which provideseffective capacitance in the 300° K. area, and curve B shows capacitancevs. temperature using only a single dielectric material, such asStrontium Titanate, which gives an effective capacitance in the 77° K.area.

Curve C shows capacitance vs. temperature for two dielectric materials,such as those identified above, co-fired in an arrangement according tothe present invention as shown in FIG. 1 and illustrating that aneffective capacitance value is provided at both the 77° K. and 300° K.areas. Curve D shows capacitance vs. temperature using the same twodielectric materials but in the particular arrangement shown in FIG. 7,termed "Piggy Back" or perhaps more accurately, a "stacked" arrangement.

While the invention has been illustrated and described with reference topresently preferred embodiments, it is understood that one skilled inthis art having the foregoing description will be able to makemodifications and changes, but it is understood also that the presentinvention is not limited to the described embodiments, but rather, theinvention is limited only by the scope of the claims appended hereto.

What is claimed is:
 1. An electrical capacitor structure for operatingeffectively as a decoupling capacitor at two different and spaced aparttemperatures, comprising:two capacitor plate means spaced apredetermined distance apart; first dielectric material means, with afirst dielectric constant to provide a predetermined capacitance valueat a first predetermined temperature, located between said two capacitorplate means and in electrical contact with both of said two capacitorplate means; second dielectric material means, with a second dielectricconstant to provide a predetermined capacitance value at a differenttemperature from said first predetermined temperature, located betweensaid two capacitor plate means and also in electrical contact with bothof said two capacitor plate means; at least one of said first and saidsecond dielectric material means is divided into multiple blocks in apredetermined arrangement between said two capacitor plate means, andsaid first and said second dielectric material means are separatedelectrically by a low modulus material as a barrier to prevent anintermingling of said dielectric material means which can create a thirddielectric material means having an unknown dielectric constant; wherebysaid electrical capacitor has predetermined values of capacitance at twodifferent and spaced apart temperatures.
 2. An electrical capacitorstructure as defined by claim 1 wherein the total capacitance valuebetween said two capacitor plate means is given by the relationship:##EQU3## where: C=the total capacitance;K₁ =dielectric constant for saidfirst dielectric material means; K₂ =dielectric constant of said seconddielectric material means; T=dielectric thickness; A₁ =electrode areawith dielectric 1; and A₂ =electrode area with dielectric
 2. 3. Anelectrical capacitor structure as defined by claim 1 including thirddielectric material means located also between said two capacitor platemeans in electrical contact with both of said two capacitor plate means.4. An electrical capacitor structure as defined by claim 1 includingthird dielectric material means located also between said two capacitorplate means in electrical parallel with said first and said seconddielectric material means, wherein said third dielectric material meansis divided into a plurality of quantities of block dielectric materialmeans stacked in a predetermined arrangement, and each of saidquantities is in electrical contact with both of said two capacitorplate means.
 5. An electrical capacitor structure for operatingeffectively as a decoupling capacitor at different temperatures,comprising:two capacitor plate means with multiple blocks of at leasttwo dielectric materials with different, predetermined dielectricconstants; each of said multiple blocks of dielectric materials beingarranged in predetermined stacks between said two capacitor plate means,and each of said multiple blocks being positioned to be in contact withboth of said two capacitor plate means, wherein a total capacitancevalue of said structure is given by the relationship: ##EQU4## where:C=the total capacitance; K₁ =dielectric constant of one dielectricmaterial; K₂ =dielectric constant of a second dielectric material; A₁=area of capacitor plate means in electrical contact with said onedielectric material; A₂ =area of capacitor plate means in electricalcontact with said second dielectric material; and T=dielectricthickness.